In C, “#ifdef” or “#if defined()” are used to hide or include portions of code only if certain conditions are met. For example, my recent *ALL RAM* BBS experiment contains code for using the SD card library as well as the Ethernet library. I used #ifdef around specific blocks of code so I could compile versions with or without either of those libraries. But all is not well in Arduino land. Consider this following, simple example:

#if defined(FOO)
byte mac[] = { 0x2A, 0xA0, 0xD8, 0xFC, 0x8B, 0xEE };
#endif

void setup()
{
Serial.begin(9600);
while(!Serial);
Serial.println("Test...");
}

void loop()
{
}

This is supposed to only include the “byte mac[] =” line if “FOO” is defined, such as with:

#define FOO

However, on the current Arduino IDE (1.0.4), this simple code will fail with:

ifdef.ino: In function ‘void setup()’:
ifdef:18: error: ‘Serial’ was not declared in this scope

What? Suddenly “Serial.println()” won’t work? Moving the byte declaration outside of the #if def make it work. Very weird.

I also found a similar example, where I tried to comment out a function that used SD library variable types:

void setup()
{
Serial.begin(9600);
while(!Serial);
Serial.println("Test...");
}

void loop()
{
}

#ifdef FOO
byte fileReadln(File myFile, char *buffer, byte count)
{
}
#endif

In this example, I did not want the fileReadln() function to be included unless I had defined FOO. But, compiling this produces:

ifdef:15: error: ‘File’ was not declared in this scope
ifdef:15: error: expected primary-expression before ‘char’
ifdef:15: error: expected primary-expression before ‘count’
ifdef:15: error: initializer expression list treated as compound expression

Unhelpful. And after wasting some time on this, I started a topic in the Arduino.cc forums to ask if others were experiencing the same thing. And they were. A helpful post from parajew pointed me to this site which helped explain the problem, and offered a workaround:

http://www.a-control.de/arduino-fehler/?lang=en

The pre-processor does some stuff behind the scenes, creating prototypes and including header files where needed, and it just does it wrong. The A-Control site figured out a simple workaround, which I trimmed a bit to just adding this at the top of my scripts:

// BOF preprocessor bug prevent - insert me on top of your arduino-code
// From: http://www.a-control.de/arduino-fehler/?lang=en
#if 1
__asm volatile ("nop");
#endif

…and now either of my examples will compile as intended. Thank you, parajew and A-Control! I can now move on to my next problem…

Hope it helps you, too.